Researchers: Carbon better than copper for TSVs

PORTLAND, Ore.—Three-dimensional chip stacks are better connected with through-silicon-vias (TSVs) filled with carbon nanotubes instead of copper, according to researchers at Chalmers University of Technology in Gothenburg, Sweden.

TSVs promise to speed up the communications among all the chips that make up an electronic system by stacking them in 3-D instead of laying them out flat on circuit boards. Unfortunately, filling the vias with copper causes problems with thermal expansion, since copper expands more than the surrounding silicon. Carbon nanotubes could solve this problem.

"Carbon nanotubes have much better properties than copper, both in terms of thermal and electrical conductivity”, said Kjell Jeppsson, a member of the Chalmers research team. "They expand about the same amount as the surrounding silicon while copper expands more, which results in mechanical tension that can cause the components to break." Other team members included Teng Wang and Johan Liu.

Nanotubes are also highly conductive and lighter weight that copper, and unlike carbon films (graphene) are easy to fabricate. To prove their point, the researcher recently demonstrated a method of filling vias with nanotubes and bonding the two chips with adhesives, resulting in both a good electrical and thermal connection betweent the stacked chips.

The remaining hurdle to mass producing 3-D chips interconnected with nanotube filled TSVs is the temperature of processing. Carbon nanotubes are usually fabricated at 700 degrees Celsius (C), whereas CMOS chips are damaged by heat much higher than 450 degrees C. However, if the last hrdle can be surmounted—such as by manufacturing the nanotubes separately and mechanically inserting them—then the researchers predict that their technique could be commercialized within five years.

Two chips have through-silicon-vias (TSVs) that are filled with thousands of carbon nanotubes. The chips are then bonded with adhesive so that the carbon nanotubes are directly contacted. A connection using two such interconnects is pictured to the right.

Yet another example of the benefits of flexible connections enabled by carbon nanotubes or graphene. It sounds like carbon is making a resurgence this year - and these solutions have the benefit of not adding any rare earths or toxic materials to the circuits.